Doubt on Priority Queue

[Muneeswaran Balasubramanian]

Hi Guys,

I have try to run the below code,In that code the result is somewhat unpredictable.

public class PriorityQueueSample
{

    public static void main(String[] args)
    {
        PriorityQueue<String> pq = new PriorityQueue<String>();
        pq.offer("Ram");
        pq.add("Raj");
        pq.offer("Ramu");
        pq.add("Ramu");
        pq.offer("Rajini");
        pq.add("Raasa");

        System.out.println("PQ : " + pq);

    }
}

Output:

PQ : [Raasa, Rajini, Raj, Ramu, Ram, Ramu]

According to which priority the elements are add in the priority queue?Please guide me.

[matej spac]

Hi, I think this thread describe similar question:

https://coderanch.com/t/535172/java-programmer-SCJP/certification/Priority-Queue-Ordering

Very valuable note in this thread from Ikpefua:

1. PriorityQueue does NOT permit insertion of non-comparable objects (doing so may result in ClassCastException).
2. The Iterator provided in method iterator() is NOT guaranteed to traverse the elements of the PriorityQueue in any particular order.

If you want selecting or printing elements from PriorityQueue in order according their priority, use peek() or poll() method.
Printing PriorityQueue elements with Iterator or like in your example only using

System.out.println("PQ : " + pq);

does not guarantee ordering.